Rocks are typically used in freshwater aquariums for their aesthetic value and to provide hiding places for the fish. Aquarium rocks provide places for fish to lay their eggs and surfaces on which algae eaters and similar scavengers can feed. Not every rock is ideal for aquarium use. Rocks must be tested before use, which is best achieved by placing them in a plastic container of aquarium water for a week. Rocks that have not leached minerals into the container are suitable for inclusion in the aquarium. The presence of minerals is detected by testing the pH of the water.
This rock is popular among fresh water aquarists, as it is hard wearing and is available in different colors. Lava or igneous rock is formed when molten lava from active volcanoes solidifies after cooling down, according to the Freshwater Aquarium Site. Lava rock should be rinsed under running water before use. The darker pieces of lava rock are often used in aquariums that house New World cichlids from the Amazon River and its tributaries. Pieces of crushed lava rock are used in the biological filter compartment of aquariums. These highly porous pieces of rock are easily colonized by strains of both aerobic and anaerobic bacteria, which make them ideal as biological filter material.
Composed of tiny grains of sand and other minerals, sandstone is a sedimentary rock that is made of feldspar and quartz. Sandstone is comes in various colors, including tan, red, yellow and white. Sandstone is resistant to weathering and frequently used as décor in freshwater aquariums. Sandstone is fairly porous and can be fragile, so aquarists need to take this into consideration when building tall structures within the aquarium.
Quartz is available in attractive shapes and in a number of different colors, including citrine, smoky and rose. A common mineral, quartz is often chosen by freshwater aquarists for their tanks. It does not alter water chemistry, as it is inert in freshwater aquariums. Composed mainly of silica, quartz is not a heavy mineral, and a number of pieces can be stacked to create caves or overhangs. Pieces of quartz are often packed against the back glass pane in an aquarium to create a natural looking wall. Large pieces of quartz can be used as individual accent features. Attractive rose quartz is a delicate pink, which highlights the colors of many freshwater fish species.
Slate is inert in the freshwater aquarium, but tends to be heavy. Silicone sealer for aquarium use can be used to secure pieces of slate together, or to combine slate with other rock types. Slate is an ideal material with which to create long and sturdy caves. Slate can be partly buried under the substrate and can form a flat foundation on top of which other types of rocks can be placed. Slate is frequently used as a surface on which fish can lay their eggs.